1. Field of the Invention
The invention relates to a substrate support chuck for electrostatically retaining a workpiece upon the support surface of the chuck. More particularly, the invention relates to a bipolar electrostatic chuck having apparatus for balancing the electrostatic force applied to a workpiece supported by the chuck.
2. Description of the Background Art
Substrate support chucks are widely used to support substrates within semiconductor processing systems. One example of an electrostatic chuck is described in commonly assigned European Patent Publication Number 0 439 000 B1, published Sep. 14, 1994. This electrostatic chuck has a conventional chuck body containing a dielectric material having a pair of coplanar electrodes embedded therein. The electrodes are half-moon or D-shaped such that each electrode provides clamping force to half the workpiece that is supported by a support surface of the chuck body.
In operation, a chucking voltage is applied to each electrode that causes an electric field to form between the electrodes. This electric field causes charge to distribute on the underside of the wafer that is oppositely polarized to charge located on the chuck surface. The Coulomb force between the charge on the wafer and the charge on the chuck surface attracts the wafer to the chuck. As such, the wafer is retained (clamped) upon the chuck surface.
Ideally, the electrostatic force that retains the wafer should be uniform across the entire underside of the wafer. However, in reality, this electrostatic force may vary substantially across the wafer during processing. The force primarily varies due to the reduction in voltage at one electrode and the increase in voltage at the other electrode caused by a bias voltage acquired by the wafer once the wafer is exposed to an RF-induced plasma. The bias voltage acquired by the wafer is the result of electrons, which are highly mobile as compared to ions also comprising the plasma, leaving the plasma and accumulating on the wafer surface, thus creating a negative charge. As the local electrostatic force is proportional to the voltage drop across each electrode and the wafer, a disparity in electrostatic force laterally across the wafer results.
For example, in a bipolar, dielectric electrostatic chuck such as that described in European Patent discussed above, the combination of the two electrodes and the wafer form, in effect, a pair of series connected capacitors. If, for example, a power source applies xc2x1400 Volts to the electrodes of a bipolar electrostatic chuck, and a plasma imparts a wafer bias of xe2x88x92100 Volts (as most electrostatic chucks are configured to be cathodes), the voltage drop between one electrode and the wafer will decrease by 100 Volts while at the electrode of opposite polarity, the voltage drop will increase by 100 Volts. This change in the voltage drop between the electrodes and the wafer due to the wafer bias results in an unequal clamping forces to be applied to each half of the wafer.
One example of apparatus used to balance these electrostatic forces is a bipolar electrostatic chuck driven by a dual power supply having a center tap. The center tap is coupled directly to a wafer residing atop the bipolar electrostatic chuck. The center tap effectively creates a feedback loop such that a change in the bias voltage on the wafer is referenced back to the power supply. As such, the voltage differential that generates the electrostatic force is maintained at both electrodes. Such an apparatus is disclosed in U.S. Pat. No. 5,764,471, issued Jun. 9, 1998, by Burkhart.
Although using the center tap coupled to the wafer provides an improvement in electrostatic force balancing in a bipolar electrostatic chuck, the connection between the center voltage tap and the wafer creates current leakage paths through the wafer. This current flow through the wafer often damages devices fabricated into the workpiece.
Another apparatus for balancing these electrostatic forces uses a third power supply at the center tap. The third power supply provides a voltage that approximately matches the wafer potential. As such, the imbalance in power supply voltages that are applied to the electrodes are compensated for. That is, an unequal voltage drop from the wafer to each of the electrodes would be adjusted by supplying a matched voltage from the third power supply. However, this type of configuration requires additional hardware (i.e., the third power supply, matching networks, and computer software/hardware I/O and the like) that are both costly and undesirable.
Therefore, a need exists in the art for apparatus and a concomitant method of automatically balancing the electrostatic force between an electrostatic chuck and a workpiece without relying on the presence of a plasma proximate the workpiece, coupling the wafer with the chucking circuit, or adding power supplies.
The disadvantages of the prior art are overcome by the present invention of a bipolar electrostatic chuck containing apparatus for balancing the electrostatic force that the bipolar electrostatic chuck imparts to a workpiece positioned upon the bipolar electrostatic chuck. More specifically, the invention is a bipolar electrostatic chuck coupled to a first and a second power supply. The electrostatic chuck contains a chuck body that is adapted to support a wafer during processing. A first electrode coupled to a first terminal of the first power supply is embedded in the chuck body. A second electrode is embedded in the chuck body. The second electrode is coupled to a second terminal of the second power supply and a first terminal of the second power supply. A second terminal of the second power supply is also coupled to ground.
In another embodiment, a method for balancing the electrostatic force that the bipolar electrostatic chuck imparts to a workpiece positioned upon the bipolar electrostatic chuck is provided. The method comprises measuring a bias voltage between a substrate and an electrostatic chuck having two electrodes, calculating an ouput voltage, and applying the output voltage to only one electrode of the electrostatic chuck.